1. Field of the Invention
The present invention is related to the field of oil well logging. More specifically, it relates to improved techniques for data acquisition in induced gamma ray logging methods.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of well logging systems seeks to measure the incidence of nuclear particles on the well logging tool from the formation for analysis purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays from the formation induced by bursts of neutrons entering the formation from a neutron source carried by the tool and pulsed at a preselected interval.
Additionally, it is frequently useful to obtain data regarding the time spectral distributions of the occurrence of the gamma rays. Such data can yield extremely valuable information about the formation, such as identification of lithologies which are potentially hydrocarbon-producing. Moreover, this desired spectral data may not only be limited to that of natural gamma rays, but also may be desired for the gamma ray spectra caused by bombardment of the formation with the aforementioned pulsed neutron sources.
Well logging systems for measuring neutron absorption in a formation use a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absorption capture cross-section Σ of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity and the quantity and type of hydrocarbons contained in the pore spaces. In wireline well logging operations, as the neutron source is pulsed and the measurements made, the instrument is continuously pulled up through the borehole. This movement makes it possible to evaluate formation characteristics over a range of depths.
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 40–60 microseconds to create a neutron population. Neutrons leaving the pulsed source interact with the surrounding environment and are slowed. In downhole environment, collisions between the neutrons and the surrounding fluid and formation atoms slow the neutrons. Such collisions may impart sufficient energy to these atoms to leave them in an excited state, from which gamma rays are emitted after a short time as the atoms return to a stable state. Such emitted gamma rays are called inelastic gamma rays. As the neutrons are slowed to the thermal state, they may be captured by atoms in the surrounding matter. Atoms capturing such neutrons are also elevated to an excited state, and emit gamma rays after a short time as the atoms return to a stable state. Gamma rays emitted due to this neutron capture reaction are called capture gamma rays.
Several prior art references are aimed at improving measurements in capture cross-section logging. For example, a method and apparatus employing a source and two detectors are discussed in U.S. Pat. Nos. 4,645,926 and 4,656,354, both issued to Randall. A subsurface instrument includes a long-spaced (LS) and short-spaced (SS) detector for detecting natural or induced gamma ray emissions from subsurface formations. The detectors produce electrical pulses, each pulse corresponding in time with the incidence of a corresponding gamma ray on the detector and having an analog voltage amplitude correlative of the gamma ray. A method is discussed in Randall '354 for determining presence of a gas by comparing first and second parameters obtained at the detectors. The first parameter is indicative of a count of detected impingements of primarily inelastic gamma radiation upon a detector. The second parameter is indicative of a count of detected impingements of primarily capture gamma radiation upon a detector. Randall '926 discusses a method of determining a parameter of the borehole, where primarily inelastic gamma radiation is normalized upon impingement on a detection device.
The number of capture gamma rays present at any time is in direct proportion to the number of thermal neutrons, i.e., the thermal neutron population. The decay rate of this neutron population is an exponential function and may be defined by specifying the time required for the thermal neutron population to decrease to one-half (“the ‘half-life time’”). Although the neutron lifetime may be measured, a more useful parameter is the capture cross-section. Capture cross-section and neutron lifetime are inversely related, the capture cross-section being a measure of the rate at which thermal neutrons are captured in the formation. Analysis of a formation using the capture cross-section or the half-life time is referred to as “neutron decay analysis.”
The well logging instrument measures the capture gamma rays to determine the thermal neutron decay time or “neutron lifetime.” The neutron lifetime represents combined effects of the thermal neutron capture cross-section in each of several volumes of space surrounding the logging instrument (“regions”). These regions generally include the instrument itself, the fluid in the wellbore, the steel casing, the cement, the earth formations radially proximal to the wellbore wall (which may have been infiltrated by fluid from within the wellbore), and the earth formations radially more distal from the wellbore wall (which have minimal infiltration from the fluid in the wellbore).
In U.S. Pat. No. 4,668,863, issued to Gray, et. al., an apparatus is used to analyze and process parameters including the macroscopic thermal neutron absorption capture cross-section of the formation at borehole elevations corresponding to the locations from which spectra are derived. For acquiring temporal spectral data, a multi-channel scale section is provided which includes a channel number generator which produces a numerical sequence of memory address codes corresponding to a sequence of adjacent time windows. Each code uniquely defines a start time. The windows collectively comprise the time interval of the desired spectrum. Each time a gamma ray pulse is detected, the tool addresses a memory location corresponding to the detection time and increments the count value resident therein. At the conclusion of the time spectrum interval of interest, the memory locations may be interrogated by the CPU and the resultant spectral data analyzed, transmitted to the surface or presented visually as a gamma ray emission count versus time plot. Correlation is made of detection signals in response to impingement of gamma radiation upon first and second detectors.
A method employed by Randall et. al. in “Time-Derived Sigma for Pulsed Neutron Capture Logging” SPE 9614, p. 345, 1981, determines the timing of the measure interval using on a two-pass Σ measurement. That is, two Σ computations are continuously derived. A first-pass Σ value is measured over the fixed-time interval from 400 μsec to 1000 μsec after each neutron burst. Simultaneously, a second-pass Σ measurement is made over a single gate of 600 μsec gate width which starts at a selectable time between 200 μsec and 400 μsec after the burst. The positioning of the second-pass gate is dependent upon the first-pass Σ value. Although positioning an acquisition window can remove the effect of noisy data points from subsequent calculation, significant noise can still be introduced.
There is a need for a technique of adjusting an acquisition window size to improve the quality of data used in gamma ray calculations. The present invention fulfills this need.